Antenna arrays are widely used in the communications field. Such antennas comprise a number of elements that form a radiation beam pattern during transmission and reception of a signal. The beam has a number of lobes and one or more nulls and the strength of the received signal applied to the receiver from the antenna, and that of the signal applied from a transmitter for transmission, depends upon the antenna beam pattern. Another factor is the angular orientation of the antenna array relative to the source of the received signal or to the target of a transmitted signal.
An array antenna operating in the receiving mode can also receive signals from interfering sources that may adversely affect the receiver. For example, consider an antenna that receives signals from two sources spaced apart, with the connected receiver being interested only in the signals from one of the sources. The signal of interest preferably would be in the main lobe of the antenna beam pattern and the signal from the other source, the interferer, would be in a side lobe. But if the signal received from the interferer in a beam pattern side lobe is strong enough, it may obscure the signal of interest during processing by the receiver. The problem increases when two or more interfering signal sources are present.
In the undersea acoustic field, a technique has been developed for a passive system, that is, one for which the signals emanate from a target, to discriminate against the interfering signal(s) emanating from a source(s) other than the one of interest. This technique uses an Interference Rejection Processor (IRP) placed between the array of acoustic transducers and the receiver. The acoustic signals are usually of a sinusoidal, continuous wave, nature and of relatively narrow bandwidth. The IRP operates to effectively process the received signal in a manner that corresponds to greatly reducing the sidelobe level of the acoustic transducer array beam pattern directed at the source of an unwanted interfering signal. Thus, the signal from the unwanted interfering target has a reduced level when applied to the receiver so that it can be more easily discriminated against, or ignored. This increases the ratio of signal power to interference power and therefore the detectability of the received signal emanating from the target of interest which is applied to the receiver for processing. The IRP technique is described in U.S. Pat. No. 4,017,867 granted Apr. 17, 1977 to Alfons Claus, which is assigned to the assignee of the subject application.
An improvement of the IRP of the Claus patent is described in "IRP (interference rejection processor) revisited - a new approach to multiple interference rejection and elimination of beam pattern distortion", by J. Minkoff, Journal of the Acoustical Society of American ("JASA"), Vol. 91, No. 2, February 1992, pages 832-843, which is incorporated herein by reference. The IRP of the Minkoff paper deals with interfering signals emanating from multiple sources at spaced locations relative to the radiation pattern of the array of acoustic transducers. It provides improved performance in discriminating against signals from multiple interference sources and elimination of the distortion in the antenna beam pattern.
Neither the IRP of the Claus patent nor that of the Minkoff paper contemplates use of the IRP for wideband radio frequency signals, such as those used in cellular communication systems, where there are a multitude of active stations at different locations that transmit radio frequency type signals to communicate with each other. This differs from the undersea acoustic use in several significant respects.
Present day cellular communication systems typically operate in the frequency range of 800 Mhz to 900 Mhz and employ pulse transmission of the electromagnetic radio frequency wave signal. Such signals differ from the sinusoidal type acoustic signals since they are wideband in nature with a typical waveform being a carrier wave modulated by an envelope consisting of a pulse train. During operation, a cell station either receives a signal to be provided to a subscriber, or transmits a signal to another cell station of the network, or to individual users of the network. The stations of the cellular system are spaced apart and operate on line of sight communication. Frequently, two or more cell stations transmitting signals can be in the line of sight of a receiving cell station. Due to the line of sight communication, a problem arises at a cell station operating as a receiver in that the radio frequency signals received by its antenna from one or more stations other than the one of interest creates an interference problem. Other sources of interference, e.g., radio transmitters, also exist.
Also, due to the relative delays between elements of the antenna array receiving radio frequency signals at a cell station, both the relative phase of the carrier wave and the pulse envelope can be shifted at the different elements of the array. Unlike the case of a pure sinusoid of an acoustic signal, where a simple phase shift by the IRP is sufficient to effect steering of the beam pattern to correct for the phase delays, it would reasonably appear by applying conventional theory that this would not be effective for a pulse train signal that is sufficiently wideband as compared to the carrier wave of a radio frequency signal. For such a wideband signal, coherent summation may require true amplitude time delay to compensate for the relative delays between the antenna array elements. This represents a very significant, and possibly, prohibitively costly, increase in system complexity.